This invention relates to a method for determining the relative movement between a target tracking missile having an image processing seeker head and a target detected by the seeker head.
In many cases it is useful to determine the relative movement of a target detected by the seeker head relative to the missile or to the seeker head. This can result in an improvement of the effectiveness of the missile.
If a high rate of direct hits is achieved by optimizing the guidance law, then the mass of the war head can be kept small. A small war head improves the radius of action of the missile. Furthermore, the manoeuverability of the missile is improved. In the case of a direct hit, the charge of explosives of the war head can be detonated by an impact fuse. However, a small war head has disadvantages when the target is missed closely. Then high demands are made on the fuse delay law, according to which the war head is triggered by means of a proximity fuse after the target has been detected According to the prior art, the approach to the target is detected by means of an active radar sensor or a laser.
An important component of the xe2x80x9cfuse delay lawxe2x80x9d is the trigger delay. This is the delay between a proximity signal generated by the proximity sensor and the actual triggering. A target is not vulnerable everywhere to the same extent. If the charge of explosives of the war head is triggered too early or too late by some fractions of a second, then the effect of the charge of explosives is not optimal. The target is not sufficiently damaged.
The optimal trigger delay depends, among other factors, on the vectorial target velocity relative to the missile and on the angle between the velocity vectors of the missile and of the target. This angle is called xe2x80x9crelative trajectory anglexe2x80x9d. Normally, these quantities are not available.
The effectiveness of a missile may also be improved by adaptation of the guidance gain in the guidance law to the relative velocity and position of the target relative to the missile.
One of the objects of the present invention is hence to estimate the relative movement between missile and target.
This and other objects are achieved by a novel method of determining the relative movement between a target tracking missile and a target. The target is located at a target distance from the missile and moves with a target velocity relative to the missile. The missile is equipped with an image processing seeker head detecting the target. The seeker head observes the target in an observation direction. When the seeker detects the target, a target image appears on the seeker with target image dimensions. The target image dimensions depend, in known manner, on the observation direction. A seeker head-fixed coordinate system is defined. A maximum absolute size of the target is defined. Further relevant quantities are measured. A recursive algorithm is run in order to obtain estimated values for a three-dimensional vector of the target velocity in the seeker head-fixed coordinate system, using as input the defined maximum absolute size of the target and the image dimensions appearing on the seeker as well as the further relevant quantities.
Some quantities can be directly measured. Such quantities are, for example, the velocity of the missile and the inertial line-of-sight angular rate, both measured in the seeker head-fixed coordinate system, as well as the remaining time of flight. The more distant the target is from the missile, the smaller is the target image with predetermined maximum absolute size of the target (in meters). Furthermore, the size of the target image depends on the direction, from which the missile observes the target, that means the so called off-tail angle OTA. At first, this off-tail angle is unknown, and so is also the distance of the target from the missile. Initial values of the unknown quantities are input together with the directly measurable quantities into a recursive algorithm. The algorithm provides estimated values for the velocity vector of the target, likewise in the seeker head-fixed coordinates. These estimated values of the velocity vector of the target and the unknown quantities are increasingly improved by the recursive algorithm.
In a preferred embodiment the method steps comprises:
(a) defining a target type and defining a correspondent maximum absolute target size;
(b) storing a table of visible absolute target sizes as a function of an off-tail angle for at least one target type;
(c) estimating the target distance by using the target image dimensions appearing in the seeker and the visible real target size at an estimated off-tail angle;
(d) determining the missile velocity, the line-of-sight angular rate and the remaining time of flight;
(e) estimate the three-dimensional vector of the relative target velocity in the coordinate system fixed to the seeker head;
(f) determining a target off-tail angle from the relative target velocity; and
(g) repeating steps (c) to (f) while using the last calculated target off-tail angle.
Further objects and features of the invention will be apparent to a person skilled in the art from the following specification of a preferred embodiment when read in conjunction with the appended claims.